Manufacture of chemically pure hydrochloric acid



pt. 3. 1940. J. w. BROWN ET AL I 2,213,544

MANUFACTURE OF CHEMICALLY PURE HYDROCHLORIC ACID Filed April 16, 193? CRUDE HYDROCHLORIC ACID oxIDIzING AGENT HYDROGEN HYDROGEN HYDROCHLtlRlC ACID VAPOR NYDRocIIIoRIc ACID mm I F E l R E CHLOR NE CATALYST HYDROGEN CRUDE IIYDRocIIIoRIc ACID PURE HYDROCHLORIC ACID FREE CHLORINE v RETORT CONDEN5ER CONSTANT- BOILING RESIDUE I IMPURITIES James WBrown Zia/lock Gfiasford INVENTORS BYAF? M ATTORNEY.

"it I PatentedSept. 3, 1940 UNITED STATES MANUFACTURE oF o EmoAL Y PURE HYDROCHLORIG ACID James W. Brown, Hammond, and Hallock C. Hosford, East Chicago, Ind., assignors to E. I.

du Pont de Nemours & Company, Wilmington, I Del., a. corporation of Delaware Application April 16, 1937, Serial No. 137,288

5 Claims. (Cl. 23-154) This invention relates to the manufacture of chemically pure hydrochloric acid and is particularly directed to processes for purifying crude hydrochloricacid wherein arsenical and organic impurities are removed by. distillation in the presence of free chlorine and wherein the effluent vapor is treated with hydrogen in the presence of a catalyst to' convert the excess free chlorine to hydrogen chloride. 7

In the production of hydrochloric acid, particularly by the well known processes involving the double decomposition of common salt (NaCl) and sulfuric acid, impurities are introduced into the evolved hydrogen chloride. A large portion of these impurities is removed during washing and filtering of the hydrogen chloride but the more volatile ones persist and are absorbed in wateralong, withthe hydrogen .chloride in the absorbers. v

For example, arsenic which is usually present in the sulfuric acid is converted into arsenous chloride which because of its low boiling point is particularly persistent. Similarly, easily volatil ized organic compounds originally present in the salt as well asorganic matter which has been converted to easily volatilized compounds by the action of. the sulfuric acid are carried over and absorbed together with the hydrogen chloride.

, These arsenical and organic impurities, for the same reasons that they persist in the crude acid, cannot be readily and economically separated by direct distillation of the crude acid. The fact that these impurities are present in the crude acid in minor amounts only, as well as the fact that the tolerances for them in chemically pure acid is inordinately small, further augments the difiiculty of their removal. For examplaarsenic which rarely exceeds one part per million in the crude acid'must be reduced toless than one tenth part per million to meet the requirements of the American Chemical Society for chemically pure acid. Similarly the tolerance for ;organic matter in chemically pure acid is very small because of the discoloration produced by small amounts of this impurity.

Altho it is known that arsenical and organic impurities may be removed from hydrochloric acid by distillation in the presence of an oxidizing agent, the'process has the disadvantage that free chlorine, liberated by the action of the oxidiz ing agent on the hydrochloric acid, is carried" over into the distillate. That this process has not been successfully adapted to commercial operation is due mainlyto the difficulty of removing' the excess free chlorine;

Many of the attempts to remove the chlorine have led' to the introduction of additional inipurities. For example, the use of acetylene or other unsaturated organic compounds has introduced chlorine derivatives. The acid is auso rrequently" contaminated with the unsaturated com pounds. I

Other suggestions, such as fractional, disti11a= tion, have proved suitable for small scale operations only. The intermittent character of this type of distillation-makes it unsuitable for come merciai ope-reacts. Fhrthermore' the first frac-' tion, that containing chlorine, is also the more concentrated one; thus the yield is materially impaired. 5 I I 15 We have found that chemically pure hydro cnl'oric' acid can be made economically and erfectively from crude acid, such as is obtained commercially from the decomposition of sulfuric acid and sodium chloride, by distilling in the presence of anoxidizing agent, more properly in the presence of an oxidizing agent, more ro erly in the presence of free chlorine which neces' sarily results from the presence of an oxidizing agent in hydrochloric acid, and treating the effluent vapor hydrogen in" tiiepresence' or a catalyst, to convert excess chlorine to hydrogen chloride. I

By these processes we are'able to produce chemically pure hydrochloric acid from crude acid in a manner consistent with co'mmercial operations; we are able to do so' effectively and economically and in a continuous manner; We are able to produce chemically, pure acid in which the arsenic and organic contents are well below the specification for chemically pure acid; and, we are able to doso without introducing other impurities.

'In the accompanying flow sheet, we have illustrated the essential steps of our process which consist briefiy" of treating the crude acid with an oxidizing agent, distilling the so-treated acid and treating the-stillvapor withhydrogen in the presence of a catalyst catalytically to convert excess free chlorine into hydrogen chloride.

Apparatus of any suitable design may be employed. Preferably, it should be designed for continuous distillation for'reasons which will be pointed out later. Briefly, operation in this manner consists in effecting acontinuous flow of 50 acid thru the retort, crude acid being introduced and the constant boiling residue being withdrawn at rates which-will give a distillate of desired concentration. I I I Distillation of the crude acid is effected in the I .blank silica carrier (i. e.,, unplatinized) presence of free chlorine in order to oxidize thearsenical impurities to less volatile forms which are retained in the still residue, and to convert organic impurities to non-volatile or non-condensible forms which are retained in the still residue or which escape to the atmosphere.

The chlorine may be introduced in several ways. It may be introduced as such, or it may be produced in the nascent state by adding an oxidizing agent to the acid. We have found that the addition of an oxidizing agent, particularly in aqueous solution, is moreeconomical and convenient. As oxidizing agents for this purpose are well known, it is sufficient to mention that among those which we have found particularly satisfactory are the alkali metal .chlorates and permanganates, and hydrogen peroxide.

The amount of oxidizing agent necessary to oxidize the impurities may be determined by gradually increasing the amount until free chlorine can be detected in the eflluent vapors or in the residue. Once this amount is determined, it may be increased sufiicientlyto cover possible variations in the analysis of the crude acid and to provide an adequate margin for safe operation. Too much can only introduce more free chlorine whereas too little may result in imperfect elimination of the arsenic. A large excess should be avoided as it is unnecessary and would only have to be removed later.

More oxidizing agent is required for intermittent or batch-type. distillation than for continuous distillation. Due to the fact that the free chlorine distills over'with the first fraction, additional oxidizing agent must be added at intervals thruout the distillation to maintain oxidizing conditions. With continuous distillation, however, the oxidizing agent is continuously added with the crude acid so that oxidizing conditions are maintained without further additions. The latter process is particularly advantageous in that the excess of free chlorine is small.

The efiluent vapor as it passes from the retort to the condenser is admixed with hydrogen and contacted with a heated catalyst to convert the excess free chlorine into hydrogen chloride. For this purpose we interpose a catalytic reaction chamber between the retort and the condenser.

This reaction chamber may be charged with any suitable catalyst capable of promoting the reaction between hydrogen and chlorine. Preferably we employ platinum catalysts such as platinum or platinum-rhodium gauze, or platinum black deposited upon a carrier. We are not limited to the use of platinum catalysts however for we have obtained satisfactory catalytic conversion with other materials. For example, a has proved entirely satisfactory.

Inasmuch as even traces of free chlorine cannot be tolerated in chemically pure hydrochloric acid and inasmuch as the concentration of free chlorine in the still vapor is small, it is essential that the catalytic mass be maintained at a temperature that will cause the reaction at a temperature above about 500 C.

Acceptable results may be'obtained at. lower temperatures (1. e., 300 c. to 500 0.) but they are not uniformly reliable; the amount of free "chlorine remaining in the distillate'istoo near the maximum limit to be considered satisfactory. (The maximum limit is that amount of free chlorine which, when the acid is treated with starch-iodide, shows the characteristic blue color on standing for ten or fifteen minutes but not before.) The lower temperatures are further objectionable because platinum catalysts are appreciably attacked by the still vapor.

Preferably the temperature of the catalytic mass should be maintained between about 650 C. and 1000. C., that is between a dark red and a bright cherry red heat. At this temperature the reaction between the hydrogen and the free chlorine is socomplete that free chlorine can. be detected in the distillate only with difficulty. Attack of platinum is also precluded. Higher temperatures may be employed but they are usually a constant flow from the exit will ordinarily be suflicient. If desired theexcess hydrogen may be recirculated. 1 i. .l In order more fully to illustrate the principle of our invention the following examples are given: a w Example 1 I In accordance with the procedure outlined above, a crude acid having the following average analysis: 1

Specific gravity degrees Bauml 22.06

Hydrogen cholride per cent 35.02 Arsenic parts per million 0.14

Organic matter High was continuously distilled at a' r'atev togive a distillate of 23 Baum. Elimultaneously'- an aqueous solution of sodium chlorate (NaClOa) was introduced at a rate such that one part by weight of sodium chlorate was added for each 1000 parts by weight of crude acid introduced.

The eiiiuent vapor admixedwith hydrogenwas then passed thru platinum gauze 'heated ito a cherry red heat.

Two runs made with this setup gave distillates of the following analyses I Run 1, Run 2 23. 20 23. 20 '37.58 37.58 0.005 '0. 01 None None Free chlorine None None The residue shows a distinct positive test'fo'r free chlorine, indicating the presence of free ch10- rine in the eiiluent vapor. I j

These data show that chemically pure acid can be produced by our processes which is not con taminated with free chlorine or organic matter and in which the content of arsenic is well below the tolerance for this impurity in chemically pure acid. It may be observed that the distillate,, as tested by the standardaalphaenaphthol testjor organic matter, failed to show a positive test for organic matter.

Example 2 Run 1 Run 2 0. 003 0. 016 None None Free chlorine None None These data show that results comparable with those of Example 1 may be obtained with other catalysts.

It is apparent that the crude acid, particularly that produced at plants having different sources of raw materials or at plants using different processes and apparatus, may vary considerablyin the nature and amount of impurities. It is also apparent that crude acids of difierent concentrations may be produced. The concentrations specified are normal trade concentrations.

We claim: v

1. The process for manufacturing hydrochloric acid which comprises distilling a crude aqueous solution of hydrochloric acid gas in the presence of traces of free chlorine whereby a mixture of water vapor and hydrochloric acid gas containing traces of chlorine is evolved, admixing hydrogen with the efliuent vapor, contacting the mixture with a catalyst to convert the free chlorine contained in the effluent vapor to hydrogen chloride, and then cooling the efiluent vapor sufiiciently to condense the mixture of water vapor and hydrochloric acid gas.

2. The process for manufacturing hydrochloric acid which comprises distilling a crude aqueous solution of hydrochloric acid gas in the presence of traces of free chlorine whereby a mixture of water vapor and hydrochloric acid gas containing traces of chlorine is evolved, admixing hydrogen with the efliuent vapor, contacting the mixture with a catalyst heated toabove 500 C. to convert the free chlorine contained in the effluent vapor to hydrogen chloride, and then cooling the effluent vapor sufiiciently to condense the mixture of water vapor and hydrochloric acid gas.

3. The process for manufacturing hydrochloric acid which comprises distilling a crude aqueous solution of hydrochloric acid gas in the presence of traces of free chlorine whereby a mixture of water vapor and hydrochloric acid gas containing traces of chlorine is evolved, admixing hydrogen with the effluent vapor, contacting the mixture with a platinum catalyst heated to above 500 C. to convert the free chlorine contained in the efliuent vapor to hydrogen chloride, and then cooling the efiluent vapor sufiiciently to condense the mixture of water vapor and hydrochloric acid gas.

4. The process for manufacturing hydrochloric acid which comprises distilling a crude aqueous solution of hydrochloric acid gas in the presence of traces of free chlorine whereby a mixture of water vapor and hydrochloric acid gas containing traces of chlorine is evolved, admixing hydrogen with the efiluent vapor, contacting the mixture with platinum gauze heated to above 500 C. to convert the free chlorine contained in the efliuent vapor to hydrogen chloride, and then cooling the efiluent vapor sufficiently to condense the mixture of Water vapor and hydrochloric acid gas.

5. The process for manufacturing hydrochloric acid which comprises distilling a crude aqueous solution of hydrochloric acidgas in the presence of traces of free chlorine whereby a mixture of water vapor and hydrochloric acid gas containing traces of chlorine is evolved, admixing hydrogen with the eflluent vapor, contacting the mixture with a blank silica carrier heated to above 500 C. to convert the free chlorine contained in the effluent vapor to hydrogen chloride, and then cooling the efiluent Vapor sufiiciently to condense the mixture of water vapor and hydrochloric acid JAMES W. BROWN. HALLOCK C. HOSFORD.

CERTIFICATE OF CORRECTION. Patent No. 2,215,5l h. September 5, 191m.

JAMES w. BROWN, AL.

It is hereby certified that error appears in the printed specification of the above numbered patent reqairing correction as follows Page 1, second column, lines 21 and 22, strike out the words and comma "more properly in the presence of an oxidizing agent,"; page 2, second column, line L5, for "cholride" read -chloride--; and that the said Letters Patent should be read with this correction therein that the same may conform tothe record of the case in the Patent Office.

Signed and sealed this 29th day of October, A. D. 19l .0.

Henry Van Arsdale, (Seal) 7 Acting Commissioner of Patents. 

